"If you'd come in a spacecraft 4.4 billion years ago, Earth would have looked a lot like it does today," said Mark Harrison, a geologist at the Australian National University in Canberra.

Harrison and his colleagues recently analyzed ancient minerals called zircons found deep in the Australian outback. They are the oldest known rocks on Earth.

Ancient metals in the zircons suggest that Earth had all the conditions necessary to make the planet habitable 4.4 billion years ago, within 200 million years of its formation, Harrison said.

But the research doesn't say life existed 4.4 billion years ago.

"Zircons don't have an opinion on that," Harrison said.

Harrison's team reported its findings last week in a paper posted on the Web site of the research journal Science.

Finding Questioned

The findings are in contrast to conventional theories that early Earth was either moonlike or dominated by oceans. Those theories hold that continental crust didn't form until 4 billion years ago, more than 500 million years after the planet formed.

Despite the findings of Harrison and his colleagues, several scientists still support the conventional theories.

Chris Fedo, a geologist at the University of Tennessee in Knoxville, asks: If the early Earth was full of continents, where are all the ancient rocks?